Headphone reproduction of sound typically provides an experience that a sound is perceived ‘inside the head’. Various virtualization algorithms have been developed which create an illusion of sound sources being located at a specific distance and in a specific direction. Typically, these algorithms have an objective to approximate a transfer function of the sound sources (e.g. in case of stereo audio, two loudspeakers in front of the user) to the human ears. Therefore, virtualization is also referred to as binaural sound reproduction.
However, merely applying a fixed virtualization is not sufficient for creating a realistic out-of-head illusion. A human directional perception appears to be very sensitive to head movements. If virtual sound sources move along with movements of the head, as in the case of fixed virtualization, the out-of-head experience degrades significantly. If the relation between a perceived sound field and a head position is different than expected for a fixed sound source arrangement, the sound source positioning illusion/perception strongly degrades.
A remedy to this problem is to apply head tracking as proposed e.g. in P. Minnaar, S. K. Olesen, F. Christensen, H. Moller, ‘The importance of head movements for binaural room synthesis’, Proceedings of the 2001 International Conference on Auditory Display, Espoo, Finland, Jul. 29-Aug. 1, 2001, where the head position is measured with sensors. The virtualization algorithm is then adapted according to the head position, so as to account for the changed transfer function from virtual sound source to the ears.
It is known for the out-of-head illusion that micro-movements of the head are most important as shown in P. Mackensen, ‘Auditive Localization, Head movements, an additional cue in Localization’, Von der Fakultat I—Geisteswissenschaften der Technischen Universitat Berlin. Yaw of the head is by far more important for the sound source localization than pitch and roll of the head. Yaw, often referred to as azimuth, is an orientation defined relative to the head's neutral position, and relates to the rotation of the head.
Today, a multitude of head tracking systems (mainly consumer headphones or gaming applications) are available which use e.g. ultrasonic technology (e.g. BeyerDynamic HeadZone PRO headphones), infrared technology (e.g. NaturalPoint TrackIR plus TrackClip), transmitters/receivers, gyroscopes (e.g. Sony MDR-IF8000/MFR-DS8000), or multiple sensors (e.g. Polhemus FASTRAK 6DOF). In general, these head tracking systems determine the head position relative to an environment, either by using a fixed reference with a stable (invariant) position relative to the environment (e.g. an infrared ‘beacon, or using the earth magnetic field), or by using sensor technology that once calibrated, does not drift significantly during the listening session (e.g. by using high-accuracy gyroscopes).
However, the known head tracking systems cannot be easily used for mobile applications in which the user moves. For such applications obtaining a positional and orientation reference is generally difficult or impossible, since the environment is mostly a-priori unknown and out of user's control.